1. Field of the Invention
The present invention relates generally to a process for the removal of niobium-rich second-phase particles (SPPs) from pickled niobium-containing zirconium alloys and a composition for use in such a process.
2. Background Information
Zirconium has many useful properties, among them good physical strength and high corrosion resistance. It is used in the chemical industry based on its ability to withstand many concentrated acids, such as nitric acid and acetic acid. In its hafnium-purified form, zirconium is widely used as a structural material for nuclear fuel cores, taking advantage of its low neutron absorption cross section. Current alloys used in nuclear grade zirconium applications typically contain tin, iron, and sometimes nickel; more recent alloy compositions such as the Westinghouse “Zirlo” alloy also contain percent quantities of niobium for improved corrosion resistance in nuclear reactor environments.
Like most metal product applications, fabrication of nuclear reactor fuel tubes and core components can leave the metal with undesirable surface features on the metal, such as scratches, oxidation stains, and chemical contamination from lubricants. Zirconium components are therefore typically pickled before use, and the parts making up nuclear fuel assemblies may be pickled numerous times during the manufacturing process to control the surface quality and remove contaminants. A typical pickling bath for zirconium can be an aggressive solution containing between 10 and 40% weight nitric acid, plus 1 to 5% hydrofluoric acid.
A specific problem may arise when pickling niobium-containing zirconium alloys. The niobium can segregate within the alloy into very small, second-phase particles (SPPs), typically having binary Zr—Nb or ternary Zr—Nb—Fe compositions. When the niobium-containing zirconium alloy is pickled, dissolution of the Zr matrix can proceed faster than that of the SPPs, so that large quantities of extremely fine, black particles are released into the pickle acid during the pickling process. Unfortunately, when the alloy is removed from the pickle acid, even after thorough rinsing, the surface can be matte black due to a dense coating of adherent particles that do not release from the metal surface during rinsing. This material is known in the industry as “smut,” a reference to the similarity of its appearance to black masses of fine fungal spores by the same name.
In general, before any niobium-zirconium alloy can be used in a nuclear reactor application, the “smut” deposits are removed; in part to yield bright, shiny product surfaces and, also to prevent later possible release of such particles into the reactor cooling water and potential deposition within the reactor. On easily accessible exterior surfaces, removal of SPP deposits is not difficult, and can be accomplished by water blasting or mechanical wiping with cloths or sponges. However, many final reactor components contain internal surfaces that are not easily accessible, such as the interior of fuel tubes for both pressurized water reactors (PWRs) and boiling water reactors (BWRs), and the interior of channel boxes in BWRs. Mechanical cleaning of some interior surfaces such as smooth cylindrical tubes may be accomplished by dragging cleaning swabs through the component, but other small channels cannot be cleaned effectively, and small crevices cannot be accessed at all.
A chemical wash, which would either dissolve the SPP deposit or release it from the metal surface, may be considered a solution to this problem. However, dissolution of Zr—Nb and Zr—Nb—Fe second phase particles may not be a likely solution, as any solvent capable of attacking the SPPs may also be capable of attacking the zirconium background even more aggressively, leading to both surface damage, and release of still more SPPs from the alloy. Development of a method and composition to release the “smut” deposit without dissolving it may depend on identification of the nature of how the particles are affixed to the surface.
Thus, an improved process for effectively removing SPPs from niobium-containing zirconium alloys without damaging the material surface is desirable.